Personally, I've never tried it but since people have mentioned this game on occasion...
As revealed by John Carmack a week ago at QuakeCon, the godfathers of the first person shooter genre, id Software, have released the source code to Quake 3 under the GPL. FileShack has the 5.46MB ZIP you will need.
http://www.bit-tech.net/news/2005/08...3_source_code/
bit-tech.net | Quake 3 source code released